1. Field of the Invention
This invention relates to a micellar slug suitable for use in a micellar drive for recovering oil from subterranean reservoirs. More specifically, it relates to a micellar slug capable of forming micro-emulsions at a high salt concentration and of readily adjusting the viscosity thereof within a wide range. The micellar slug of this invention is especially suitable for use in a micellar drive for recovering oil from subterranean reservoirs mainly formed by alkaline rocks including alkaline earth metal carbonates.
2. Description of the Prior Art
It is well known in the art that the so-called "primary recovery" methods, including pumping methods, can recover only a portion of petroleum or crude oil (which is referred as "oil" herein) from subterranean reservoirs and leave substantial amounts of oil in the subterranean reservoirs.
In order to recover the remaining large amounts of oil from the subterranean reservoirs, the so-called "secondary recovery" methods have been proposed. For example, water or gas is injected into subterranean reservoirs from an injection well at a pressure sufficient to increase the flowability of the oil, steam is injected into subterranean reservoirs so as to effect the displacement of oil toward a production well, or oil in subterranean reservoirs is partially burned to heat the subterranean reservoirs so as to decrease the viscosity of the oil and increase the flowability of the oil. Thus, by means of these methods, oil is recovered from subterranean reservoirs. Furthermore, the so-called various "tertiary recovery" methods, including a combination of secondary recovery methods and improved secondary recovery methods, utilizing surfactants or water-soluble polymers have also been proposed. These methods are generally called "enhanced oil recovery" (EOR) methods.
Of these EOR methods, the recent "micellar drive" methods are to be noted. According to these methods, a micellar slug, that is, clear micro-emulsion derived from water and oil such as petroleum, petroleum distillates or fuel oil, is injected under pressure into subterranean reservoirs for the recovery of oil in the subterranean reservoirs. These EOR methods are disclosed in, for example, U.S. Pat. Nos. 3,506,070, 3,613,786, 3,740,343, 3,983,940, 3,990,515, 4,017,405, and 4,059,154. These prior arts disclose that various kinds of surfactants including anionic-, nonionic-, and cationic-type surfactants can be used alone or in any mixture thereof in the formation of micellar slugs. Examples of such surfactants are petroleum sulfonates, alkylaryl sulfonates, dialkyl sulfosuccinates, alkane sulfonates, polyoxyethylene alkylether sulfates, alpha-olefin sulfonates, polyoxyethylene alkylethers, polyoxyethylene alkylphenylethers, polyol fatty acid esters, alkyltrimethyl ammonium salts, and dialkyldimethyl ammonium salts.
These surfactants used in the formation of micellar slugs must be available at a low cost since a large amount of micellar slugs is used for the recovery of oil from subterranean reservoirs. The surfactants used in micellar slugs should have a good thermal stability, salinity tolerance, and hard-water resistance because, since numerous oil production wells or oilfields are present in the world, subterranean reservoirs have a variety of properties and also a variety of available water, for example, from soft water containing no substantial amount of inorganic salts to brine containing large amounts of inorganic salts and polyvalent metallic ions. Furthermore, as mentioned above, the micellar slugs desirably have a viscosity approximately equal to that of the oil remaining in the subterranean reservoirs. Accordingly, it is desirable that the viscosity of micellar slugs be capable of being readily adjusted depending on the properties of the oil production wells. Since oilfields subjected to EOR methods usually contain oil having a high viscosity, the use of micellar slugs having a high viscosity in EOR methods is desirable.
It is known in the art that petroleum sulfonate is an optimum surfactant usable as an injection fluid in a micellar drive, especially due to its availability and low cost, since a large amount of an injection fluid is used in a micellar drive. However, a problem exists in that petroleum sulfonate has an unsatisfactory salinity tolerance, hard-water resistance, and viscosity adjustability, and, therefore, it can be applied only in the case of special oilfields. For this reason, various attempts have been made to improve the properties of petroleum sulfonate by using petroleum sulfonate together with other surfactants or water-soluble polymer-thickening agents. However, micellar slugs or surfactants having the desired properties and a satisfactory cost have not been obtained. Furthermore, when rocks constituting oilfields are formed by alkaline earth metal carbonates, the surfactants usable in such oilfields are naturally limited.